The amount of DNA in a sample has traditionally been measured either by spectrophotometric means or fluorometrically with the use of ethidium bromide. If the sample is pure (does not contain significant amounts of contaminants such as protein, phenol, agarose, or other nucleic acids) the spectrophotometric measurement of the amount of ultraviolet (UV) irradiation absorbed is simple and accurate. However, if there is contamination with protein or compounds which absorb strongly in the UV, such as phenol, accurate quantitation of the amount of DNA will not be possible. Furthermore, this technique is only suitable for samples containing DNA in the .mu.g/ml range.
If the amount of DNA in the sample is small, or if the sample contains significant quantities of impurities, the amount of DNA may be estimated from the intensity of UV-induced fluorescence emitted by ethidium bromide intercalated into the DNA. The amount of fluorescence is proportional to the total amount of DNA. The quantity of DNA in the sample therefore can be estimated by comparing the fluorescent yield of the sample with that of a series of standards. As little as 1 to 5 .mu.g/ml of DNA can be detected by this method. With the use of a mini-fluorometer (such as that manufactured by Hoefer Scientific Instruments, San Francisco, Calif.) and the fluorochrome Hoechst 33258, the sensitivity may be increased to 10 ng/ml.
With the advent of recombinant DNA technology, it has become imperative to be able to identify significantly lower concentrations of DNA in a sample, for example, any contaminating DNA which may be present in a recombinant product. The contaminating DNA may be non-specific and of unknown sequence. Therefore, enzyme amplification of sample DNA (using for example the DNA polymerase chain reaction method) is difficult for lack of universal primers for DNA synthesis. There is, therefore, substantial interest in being able to detect rapidly and accurately the presence of extremely small amounts of DNA.